Primary and secondary (rechargeable) manganese dioxide-based alkaline cells are well known and include a positive electrode having manganese dioxide as an active material, a negative electrode utilizing zinc as the active material, an aqueous solution of potassium hydroxide as electrolyte, and a separator between the positive and negative electrode.
To overcome recharge problems of the MnO2 in the positive electrode, cells were developed in which the discharge capacity of the cell was limited by imposing a zinc electrode limitation. Due to problems with the rechargeability of the MnO2 cathode, these cells experience capacity fading on deep discharge, which results in a successive reduction of the available discharge time following each discharge-charge cycle. End-users of the batteries perceive this as diminishing usefulness and may be inclined to prematurely dispose of the battery. The occurrence of capacity fade is evidence that the manganese dioxide electrode is not fully reversible. Numerous approaches have been taken to reduce the capacity fade experienced such as employing various additives to the positive and the negative electrodes.
In this regard, reference is made to Kordesch et al in German patent number 3,337,568 issued Apr. 25, 1984. This patent describes a method for producing electrolytic manganese dioxide that is doped with titanium. Such a titanium-doped MnO2 is particularly suitable for use in rechargeable manganese dioxide/zinc cells.
Taucher et al, in WO 93/12551 filed Dec. 21, 1992, discloses improvements to primary and rechargeable alkaline manganese dioxide cells, containing barium compounds in an amount of 3-25% of the MnO2 positive electrode material.
Tomantschger et al, in U.S. Pat. No. 5,300,371, issued Apr. 5, 1994, teaches a rechargeable alkaline manganese dioxide cell with improved performance and cycle life containing organic binders, and silver and barium compounds added to the MnO2 positive electrode.
Daniel-Ivad, et al., in U.S. Pat. No. 6,361,899 discloses a range of additives to the positive electrode formulation comprising a first additive selected from the group consisting of barium and strontium compounds; and, a second additive selected from the group consisting of titanium, lanthanum, cerium, yttrium zinc, calcium, tin and magnesium compounds.
Daniel-Ivad, in U.S. Pat. Appl. No. 2005/0164076 teaches the use of hydrophobic additives for more efficient processing of cathode pellets comprising hygroscopic additives such as oxides, hydroxides, or hydrates of barium or strontium. These hygroscopic additives desirably increase the performance of the cell, as indicated by increases in the cumulative discharge capacity and cycle life of the cell.
While the foregoing references disclose a number of approaches for improved performance and cycle life, rechargeable alkaline cells still show capacity fade and diminishing useful capacity as the number of cycles increase. Particularly at moderate to low rates of discharge at 10-15 mA/cm2, the capacity fade is still very pronounced.
Accordingly, there is still a need for an improved rechargeable alkaline battery cathode composition that results in increased overall battery performance and cycle life.